This invention relates to a sealed bearing assembly for an internal combustion engine and, more particularly, to a sealed bearing assembly for supporting a crankshaft of a two-stroke engine.
In a conventional internal combustion engine, the crankshaft is rotationally supported by bearings positioned in the crankcase. In a conventional four-stroke automotive engine, these bearings are usually of the sliding (bushing) type lubricated by a pressurized oil feed. In a conventional four-stroke engine as commonly used in motorcycles, ATV""s, etc., the crankshaft is often supported by roller bearings that are lubricated by either a pressurized oil feed and/or oil spray from the crankcase. In either case, the crankcase is isolated from the combustion chamber, as is conventionally known, so the crankshaft bearing lubricating oil is kept isolated from the combustion chamber. Separate grease seals are often positioned outboard of the bearings to provide sealing between the crankshaft and an exterior of the crankcase. In recent years, environmental considerations have dictated that exhaust emissions be reduced in internal combustion engines. In a four-stroke engine, such efforts concentrate on the combustion process, since the crankcase lubrication is isolated from the combustion process and does not increase exhaust emissions in a well-maintained engine.
This is not the case for a two-stroke engine. In a conventional two-stroke engine, as used in motorcycles, snowmobiles, personal watercraft, etc., the crankshaft is usually supported by roller bearings with separate seals mounted outboard of the bearings to seal in both the lubrication and the intake charge present in a conventional two-stroke crankcase. The roller bearings can be lubricated by oil mixed in with the intake gas charge that enters the crankcase and/or by oil injection directly to the bearings. However, any oil entering the crankcase of a conventional two-stroke will enter the intake charge and be forced into the combustion chamber through the transfer ports, as is known. Thus, this oil will be burned in the combustion process, thereby increasing exhaust emissions. As a result, decreasing exhaust emissions in a two-stroke engine necessarily requires efforts with respect to both the combustion process and the crankcase lubricating oil.
Certain approaches to limiting the amount of lubricating oil entering the crankcase in a two-stroke engine have proven somewhat successful in reducing exhaust emissions. In one approach, the amount of lubricating oil supplied to the intake charge is merely reduced, thereby reducing the amount of oil which does not come into contact with any lubrication requiring surface but which is merely burned during the combustion process. The problem with this approach is that as the amount of lubricating oil is reduced, the probability of inadequate bearing lubrication increases, especially in engines having high power output, with predictable undesirable results.
Other efforts generally concentrate on reducing the amount of oil supplied to the intake charge, but separately supplying smaller additional quantities of oil directly where it is needed to compensate for the leaner concentration of oil in the intake charge. For instance, the amount of oil required by the crankshaft bearings can be directly injected to those bearings while simultaneously reducing the amount of lubricating oil supplied to the intake gas charge. Thus, the overall amount of lubricating oil used is reduced, along with the exhaust emissions from burning the oil. On the other hand, the lower limit of the amount of lubricating oil supplied to the crankcase (and thus, the combustion chamber) will depend on the lubrication requirements of the engine components, such as the crankshaft and rod bearings, pistons, etc. In any event, the oil supplied to the crankcase will either be burned during the combustion process or exit in the exhaust as unburned hydrocarbons. In recent years, as technical advances have resulted in increased power outputs from two-stroke engines, existing crankshaft bearings have been less able to withstand this increased loading over time, especially as lubricating oil has been reduced to lower exhaust emissions.
One proposal to limit the free lubricant supplied to the intake charge is given in U.S. Pat. No. 3,641,990 to Kinnersly, issued Feb. 15, 1972. Kinnersly discloses the use of a one-sided crankshaft supported by a pair of crankshaft roller bearings enclosed on either end by seals, with an annular chamber disposed between the two roller bearings filled with lubricant at the time of engine assembly. Kinnersly discloses that such an arrangement can be used with either a two-stroke or a four-stroke engine and that the roller bearings and seals can be separate or integral units. There is no disclosure as to what type of seal should be used or how such a seal would be integrated into the roller bearing. In a four-stroke engine, this is less significant because the crankcase pressures are generally not high enough to interfere with the seals. It is also clear that with the use of the one-sided crankshaft, that the Kinnersly engine is not intended for high performance and the severe loadings resulting therefrom.
Furthermore, conventional sealed bearings cannot be used to support the crankshaft in a two-stroke engine. Such bearings usually use a grease seal having a single sealing lip contacting the inner race of the bearing to provide the sealing engagement. While such sealed bearings are capable of sealing the lubricant in the bearing, they are not effective at withstanding the pressure pulses in the crankcase and preventing the intake charge from escaping the crankcase. FIG. 4 shows such a sealed bearing 230 having an inner race 232, an outer race 234 and a plurality of roller elements 236 separated by a cage 238 disposed between the inner and outer races. A seal 240 having a narrow outer edge 242 is mounted in a counterbore 244 in outer race 234. The narrow outer edge 242 engages a groove 246 in the counterbore 244 to be retained in the bearing 230. Since the outer diameter of the outer edge 242 of seal 240 is greater than the outer diameter of counterbore 244, the seal 240 must be axially flexed to reduce the outer diameter of the seal so that the narrow outer edge 242 can engage the groove 246. The seal 240 has a single lip 248 backed by a circumferential spring 250 for providing a sealing engagement with the inner race 232. The axial flexibility of the seal 240 (necessary for installing the seal in the outer race), as well as the single lip 248 do not provide the desired effectiveness in preventing crankcase pressure loss.
While seals having two separate sealing lips for sealing the crankshaft/crankcase of a two-stroke engine are known, they have previously been used only as separate seals mounted outboard of the innermost crankshaft support bearings. See FIG. 3, which shows a partial section of a prior art two-stroke engine 10 of the assignee of the present invention where a separate seal 200 is mounted outboard of conventional crankshaft support bearing 202, which is similar in design to conventional crankshaft support bearing 46. The seal 200 has a narrow outer edge 204 that engages a groove 206 in the crankcase 16 to maintain the position of the seal. The seal includes a first circumferential sealing lip 208 and a spaced apart second circumferential sealing lip 210, the sealing lips backed by circumferential springs 212 and 214 respectively.
Therefore, the present invention is a sealed bearing suitable for use in a crankcase of an engine and, more particularly, for supporting a crankshaft in a high output two-stroke engine. The sealed bearing assembly includes an inner race and an outer race with a plurality of roller elements (bearings) disposed between the inner and outer races. The outer race includes a counterbore to one side of the plurality of roller elements along an axis of the bearing assembly with a bottom ledge of the counterbore spaced axially outward from the roller elements. A grease seal (or oil seal) is disposed between the inner and outer races and positioned in the outer race counterbore with the bottom ledge of the counterbore preventing the grease seal from contacting the roller elements when the grease seal is installed in the outer race.
The grease seal includes a rigid support structure and a flexible sealing member attached to the rigid support structure. The flexible sealing member includes first and second circumferential sealing lips spaced apart along the axis of the bearing, with each of the circumferential sealing lips contacting a surface of the inner race to provide a sealing engagement between the grease seal and the inner race. These two sealing lips increase the sealing capability of the bearing assembly. Thus, the grease seal retains lubrication in an interior of the bearing and prevents lubrication from escaping to the interior of the engine crankcase. The grease seal also prevents gases from the crankcase from escaping through the bearing due to crankcase pressure variations in the engine. The other side of the bearing can also be sealed as described above with lubricant disposed between the two seals, or in a preferred embodiment, open to a reservoir of lubricant. In a preferred embodiment, the flexible sealing member is constructed of Viton(copyright).
In addition to reducing the lubrication in the intake charge, and thus, the objectionable exhaust emissions, the present invention also provides other benefits. First, since the lubrication is permanently supplied to the crankshaft bearings, the crankshaft bearings are properly lubricated upon start-up of the engine. This reduces damage to the bearings upon start-up, as can happen in an engine with conventionally lubricated bearings, especially if the engine is subjected to high loading prior to sufficient lubrication reaching the bearings. This is also important when the engine is used in applications where very low temperatures are encountered, such as in snowmobiles, and bearing clearances upon start-up are reduced due to the low temperature. The permanent lubrication prevents damage to the bearings under such conditions. This permanent lubrication can also prevent bearing damage even when the engine is warmed-up in situations where the lubrication to the bearings would otherwise be insufficient for the load experienced. Second, the permanent lubrication to the bearings can reduce the amount of lubricating oil required to be supplied to the intake charge, thus reducing the frequency of topping up the oil in the vehicle (in engines where the oil is injected into the intake stream, as opposed to premixed). Third, a broader array of lubricating oils and/or greases can be utilized for lubricating the bearings since no consideration need be given to the combustibility of the lubricant, its tendency to foul spark plugs or to leave unwanted deposits in the combustion chamber or exhaust. Thus, greater flexibility is provided when selecting a lubricant, especially when selecting a lubricant suitable for special operating conditions, such as very low temperature operation.
Therefore, it is an object of the present invention to reduce the amount of engine lubrication supplied to an intake charge of a two-stroke engine that enters the combustion chamber of the engine.
It is a further object of the present invention to reduce objectionable exhaust emissions of a two-stroke engine due to lubrication in the intake charge.
It is a further object of the preset invention to assure that the crankshaft bearings are properly lubricated under all normal operating conditions.
It is a further object of the present invention to provide more flexibility when selecting lubricants for lubricating the crankshaft bearings.
It is a further object of the present invention to provide a sealed crankshaft bearing assembly for a two-stroke engine effective at sealing lubrication in the bearing, as well as sealing against pressure loss from the crankcase.